1. Field of the Invention
The present invention relates to an image forming apparatus that employs an electrophotographic recording process to form an image. Examples of such image forming apparatus include a laser printer, a copying machine, a facsimile machine, of a multifunction printer (MFP).
2. Description of the Related Art
In know image forming apparatuses that use the electrophotographic recording process, there is a phenomenon called “white gap”. White gap is an undesirable printing artifact, in which a white gap appears which should not be present, between images formed adjacent to (or overlaid on) each other in different colors. The phenomenon occurs resulting from the fact that an electrostatic latent image, for example, an image edge portion where drum surface potential steeply changes is formed, on a photosensitive drum, and when this region is developed by a development device, a visualized image is formed thinner than originally intended. For example, in an image where a band of cyan color and a band of black color are made adjacent to each other, the band of cyan and the band of black ought to be formed adjacent to each other without a blank space therebetween, but respective visualized images are formed thin, and a gap (blank space) is produced between the cyan and black colors in a final image formed on a recording material. This type of undesired artifact is known as “white gap”, but in generally refers to a blank space.
FIG. 17 is a diagram illustrating simplified details of the white gap phenomenon pertaining to conventional technology. Specifically, FIG. 17 illustrates a state of an electric field (E-field) interaction between a development roller and a photosensitive drum during an electrophotographic recording process. Thinning of the image of visualized image portion, which causes the white gap, is attributable to winding up of the electric field (winding E-field) at an edge portion of the electrostatic latent image in an electrostatic portion formed on the photosensitive drum.
To address this problem, there is known a method for preventing thinning of an image by causing a light-emitting element of a laser scanner to emit weak light which is not enough to cause the toner to adhere, on a non-image portion (non-toner image forming portion) in the whole surface of printable region. Hereinbelow, the method is referred to as background exposure, or weak light emission for non-image portion.
The object of performing weak light emission for a non-image portion is not limited to prevention of the white gap. As discussed in Japanese Patent Application Laid-Open No. 2003-312050, the weak light emission for a non-image portion is also carried out as a measure to decrease transfer potential contrast, and to prevent an image distortion induced by an aerial discharge which is generated at transfer nip portion. In other words, the weak light emission for non-image portion is not limited to a specific purpose.
As a specific approach for the weak light emission for non-image portion, for example, a method called a pulse-width modulation (PWM) process for changing a duty cycle of a pulse wave is discussed in Japanese Patent Application Laid-Open No. 2003-312050. In this method, the light-emitting element of a laser scanner is caused to emit light in the non-image portion at a pulse-width corresponding to a weak light emission amount in synchronization with an image clock which shows a fixed frequency.
In recent years, higher image quality has been required in color image forming apparatuses. Under such situation, in addition to an adjustment of light emission amount corresponding to the image portion, appropriately adjusting a light amount in the weak light emission of the non-image portion described above becomes important. In other words, it has become important to make an appropriate adjustment to enable one light-emitting element to emit two light emission intensities.